Module Overlap

Table of Contents

• Aim of module
• General description
• Parameter description
• Examples
• References

See also

opals::IOverlap

Aim of module

Analyses a set of strip boundaries and determines all overlapping strip pairs.

General description

Checking the fitting accuracy of flight strips is of crucial importance for ALS data processing. Respective accuracy measures (like vertical and/or planar discrepancies) can be derived for the common (=overlaping) area of two flight strips. On the basis of a given set of input files (parameter inFile), each containing the boundary of a single ALS flight strip (c.f. Module Bounds), Module Overlap determines all overlapping strip pairs by sequentially intersecting all strip boundary polygons. All strip pair names sharing a common area are written to a text output file (parameter outFile) for later use in subsequent processing steps. By default, all overlapping pairs are considered. However, a minimum overlap area (parameter minArea) can be specified in order to exclude pairs with minor overlap. Relative specifications in percent and absulute measures can be combined freely with logical operators AND/OR (c.f. below for details). Optionally, additional shape file output of the polygon(s) describing the entire project area covered by all strips (-borderFile) and/or the outlines of all overlapping strip pairs (-overlapFile) is supported.

The intended use of Module Overlap is:

• Run Module Bounds with all flight strips
• Run Module Overlap with all flight strips boundaries as input
• Run Module Diff with all strips pairs (i.e. output of ) Module Overlap

Parameter description

-inFilestrip boundary files (SCOP Winput)

Type: opals::Vector<opals::Path>
Remarks: mandatory
The strip boundary files are expected in SCOP Winput or ESRI Shape file format. Alternatively, ODM files may also be specified, in which case the associated boundary files (*_bounds.shp) are considered.

-outFileoutput text file (strip pair list)

Type: opals::Path
Remarks: default=stripPairs.txt
The output file (ASCII) receives the list of all overlapping strip pairs. Each line contains one strip pair in the following syntax (strip_file_1 | strip_file_2).

-minAreaminimum overlap area

Type: opals::OverlapDef
Remarks: optional
If specified, a potential overlapping dataset pair is only considered for the output, if the minimum overlap area is beyond the given measure. The minimum area can be specified in relative and/or absolut units. Relative mesures have to be postfixed with the percent sign (%). Relative and absolute measures can be combined by AND/OR. Example: -minArea "15% AND 50000". The percentage of the overlapping area is calculated w.r.t to both contributing boundary polygons and the pair is added to the output list only if the lower of both percentages is grater than or equal the specified area threshold.

-borderFileoverall boundary file path (.shp)

Type: opals::Path
Remarks: optional
File containing the union region of all strips in ESRI Shape file format.

-overlapFileoverlap file path (.shp)

Type: opals::Path
Remarks: optional
File containing the boundary polygons of all overlaping strip pairs in ESRI Shape file format.)

-pairListstrip pair list

Type: opals::Vector<opals::Array<opals::Path, 2>>
Remarks: output
List (vector) of all overlapping strip pairs.

Examples

All data used in the following examples are located in the $OPALS_ROOT/demo/ directory of the OPALS distribution.

Prerequisites:

opalsImport -inFile strip11.laz
opalsImport -inFile strip21.laz
opalsImport -inFile strip31.laz
opalsBounds -inFile strip11.odm
opalsBounds -inFile strip21.odm
opalsBounds -inFile strip31.odm

A list of all overlapping strip pairs is obtaind by:

opalsOverlap -inFile strip11_bounds.shp strip21_bounds.shp strip31_bounds.shp

Since no specific output file has been specified, the default file =stripPairs.txt is generated with the following contents:

strip11_bounds.shp | strip21_bounds.shp
strip11_bounds.shp | strip31_bounds.shp
strip21_bounds.shp | strip31_bounds.shp

The file contains all overlapping strip pairs, i.e. all possible combinations in this special case, regardless of the overlap size. To ensure a minimum overlap area of 15%, the following command is used:

opalsOverlap -inFile strip11_bounds.shp strip21_bounds.shp strip31_bounds.shp -minArea 15% -outFile stripPairs_minArea1.txt

The minimum area can be specified in percent, absolute numbers or in any combination of both. To use only those pairs featuring an overlap area of at least 15% and an absolute area of at least 50000 square units, the following command is appropriate:

opalsOverlap -inFile strip11_bounds.shp strip21_bounds.shp strip31_bounds.shp -minArea "15% AND 50000" -outFile stripPairs_minArea2.txt

References

Mandlburger, G., Otepka, J., Karel, W., Wagner. W. and Pfeifer, N.,

1. Orientation and Processing of Airborne Laser Scanning data (OPALS) - concept and first results of a comprehensive ALS software. ISPRS Workshop Laserscanning 2009, Paris, France.